1. Technical Field
This invention relates to dry dock systems for use in lifting vessels out of the water for maintenance or repair purposes. Typically these types of docks can lift anything from one to several hundred tonnes.
2. Related Art
There are basically two types of dry dock. There are those comprising a lock that has at least one closable door into which the vessel is floated, and the water is drained from the lock to leave the vessel high and dry.
A second type of dry dock system comprises a floating dock that consist of a raft that is floated to a region ahead or astern of the vessel and submerged so as to be positioned beneath the vessel. The raft has floatation chambers built into the walls of the raft so that they can be purged of water by displacing the water with compressed air. A major problem with this type of dock is that the amount of required “water plane” makes these types of docks highly unstable. “Water plane” is defined as the area of water at the water air interface which is displaced by a part of the dock. In general the greater the “water plane” the greater will be the stability of the dock. As these docks lift a boat out of the water, there is considerable “water plane” provided by the engagement of the boat hull with the water, but it becomes particularly dangerous as the “water plane” decreases when the hull is lifted out of the water and eventually loses contact with the water. As the boat leaves the water this adds considerable weight to the dock with a considerable and rapid decrease in the “water plane” making the whole system extremely unstable, in the final stages of the lifting operation.
To remain within the bounds of stability, it is traditional to design the dock system so that it lifts vessels of about one half of the weight of the dock itself.
There is a need to provide dry dock facilities for small boats at local harbours, moorings, club harbours or lagoons and the like. There is also a need for providing a much cheaper design of floating dock than has been possible before and one that is easily moveable from one location to another. There is also a need to be able to produce a dry dock system that can be used to lift vessels out of the water rapidly thus saving valuable time and cost. Conventional dry dock systems do not permit the rapid lifting of vessels because of the problems due to the unstable designs associated with the “water plane” problem mentioned above.